In a fiber optic gyroscope a clockwise (CW) beam and a counterclockwise (CCW) beam pass around a coil of optical fiber and are brought together on leaving the coil to interfere on a detector. When rotation is applied to the gyroscope a rate-related non-reciprocal phase shift is introduced between the CW and CCW beams which varies the intensity on the detector. In some closed loop designs of such gyroscopes, a further compensatory, non-reciprocal phase shift is applied to "null" the rotation-induced phase shift and the applied rate is then determined on the basis of the amount of the compensatory phase shift required. In these designs, the intensity of the signal on the detector is used in the gyroscope control loop but is not used as an absolute measure of the applied rate. However, the processing of such closed loop gyroscopes means that they are complex, requiring precision modulators and are thus expensive to produce.
In an open loop gyroscope, the applied rate is determined on the basis of the variation in intensity of the combined CW and CCW signals and so accuracy in measurement of the intensity is very important if the gyroscope is to provide accurate readings over a large dynamic range.
Thus, a need exists for an open loop fiber optic gyroscope which implements a processing technique which allows accurate determination of the variations in intensity due to applied rate, preferably reducing some of the errors associated with relatively low cost phase modulators and offset errors in general.